The present invention relates to a process for the production of precipitated calcium carbonate from a calcium carbonate-rich by-product generated in an industrial process, specifically, a by-product generated by a nitrophosphate plant in the fertilizer industry.
Precipitated calcium carbonate finds varied commercial uses, including the manufacture of paper, rubber, plastics, glass, textiles, putties, chalks, sealant, adhesives, paints, inks, varnishes, food, cosmetics, dentrifices, chemicals and pharmaceuticals.
Commercial applications of precipitated calcium carbonate require well-defined powder characteristics, particularly, fine particles with a narrow size distribution, uniform shape and crystallinity. Marentette J. M. et al. (xe2x80x9cCrystallization of Calcium Carbonate in the presence of PEO-block-PMAA copolymersxe2x80x9d, Adv. Mater., 9, 647, 1997) have shown that these characteristics play a crucial role in product properties and that their control is important for the preparation of industrially useful products. Precipitated calcium carbonate must also be substantially free of impurities to be useful for various commercial applications. Several physical and chemical processes have been reported for the treatment of solid waste containing calcium carbonate.
Physical processes require drying and grinding to a fineness that allows impurities to be removed by screening, classification, magnetic separation, hydrocyclone and floatation separation. The disadvantages of these physical methods include the requirement of special equipment and the required maintenance thereof. Other disadvantages of existing processes include unpredictable process efficiencies, variable results in the quantities of impurities removed and the expenses associated with employing such physical techniques.
Chemical processes for the purification of calcium carbonate-rich waste involve leaching or bleaching of impurities using special reagents. Disadvantages of these methods include the requirment of a number of unit operations to perform the treatment. Moreover, a single chemical process may not be suitable for removal of all the impurities. Removal and separation of excess chemical reagents after the treatment is another disadvantage.
Another previously known and widely used method for the treatment of calcium carbonate-rich waste is reburning within the kiln and removing the reburned waste after cooling to obtain calcium oxide. The calcium oxide is then recycled in a causticizing process in producing paper pulp or it is subjected to hydration followed by carbonation to produce precipitated calcium carbonate.
In the process of Richard Woode (U.S. Pat. No. 4,018,877), an aqueous suspension of calcium hydroxide at 25xc2x0 C. was agitated vigorously and reacted with a mixture of air and carbon dioxide. After 15 minutes (following the xe2x80x98primary nucleation stagexe2x80x99) a complex-forming agent, such as a hydroxy carboxylic acid, particularly a hydroxy poly-carboxylic acid (, e.g., citric acid and malic acid) which complexes calcium ions. The complexing agent was added in a concentration range of 0.001 to 5 wt. %, preferably in the range 0.03 to 0.2 wt. % based on the weight of the calcium carbonate produced. The carbonation was stopped after about 50 minutes when the reaction mixture had just become acid to a phenolphthalein indicator. The mixture was then heated to 85xc2x0 C. over a period of 20 minutes and was allowed to age for 30 minutes. Carbonation was restarted at a much lower rate, maintaining the temperature at 85xc2x0 C. After 20 to 40 minutes the pH of the batch had fallen below 8.0. At this stage, 0.8% stearic acid in ammonia solution was added and the mixture was stirred at 85xc2x0 C. for about 3 hours. The suspension was filtered. The filter cake was extruded through {fraction (5/16)} inch diameter holes to yield xe2x80x9cgranulesxe2x80x9d which were dried in an oven overnight at 130xc2x0 C. on a gauze-tray to produce calcium carbonate having 0.72 relative granule hardness and 0.07 micron ultimate particle size with a soft texture. The drawbacks of this process are that the total batch/production time is more than 5 hours during which time the temperature is maintained at 85xc2x0 C. for a period of 4 hours. In addition, the process requires drying of the product overnight at 130xc2x0 C. This process is thus highly energy consuming and is therefore unattractive.
Hiroji Shibazaki et al. (U.S. Pat. No. 4,133,894), disclose that precipitates of uniform particle size can be continuously produced by repeating the step of carbonation reaction. In the first step of the process, a suspension of calcium hydroxide having a solids concentration 0.1 to 10 weight % and a temperature of 15 to 30xc2x0 C. is sprayed in the form of droplets of about 0.2 to 1.0 mm in diameter against a gas containing 10 to 40 volume % carbon dioxide in countercurrent contact therewith. The gas is passed at a specified superficial velocity of about 0.02 to 0.5 m/sec. By this process, 5 to 15% of the calcium hydroxide is converted to calcium carbonate. In the second step of this process, the suspension resulting from the first step is sprayed in the form of droplets of about 1.0 to 1.5 mm diameter against a gas containing 15 to 35 volume % of carbon dioxide and passed upward through the column at a superficial velocity of about 1.5 to 2.5 m/sec whereby growth of crystals is accomplished. In the third step of this process, the suspension resulting from second step is sprayed at a temperature of up to 30xc2x0 C. in the form of droplets of about 1.5 to 2.0 mm in diameter into a column in countercurrent contact at a superficial velocity of about 1.5 to 3.0 m/sec whereby the carbonation is completed. Thus, superfine calcium carbonate having an average particle size of less than about 0.1 to 3.0 microns is produced. The main drawback of this process is that it requires control of number of parameters such as solids concentration, droplet size, temperature of suspension, gas velocity of carbon dioxide containing gas etc. for three columns. Another drawback is the requirement of multi-step carbonation which is more expensive in terms of operating cost for columns and pumps than a single stage carbonation.
Bleakley, Ian S. et al. (U.S. Pat. No. 5,342,600) describe a method of preparing precipitated calcium carbonate which comprises: (1) slaking quick lime in an aqueous medium, (2) subjecting the aqueous medium to continuous agitation during said slaking, (3) passing a suspension of calcium hydroxide obtained after slaking through a sieve having an aperture size of 40-70 microns, (4) subjecting the suspension to high energy high shear agitation with an impeller having a peripheral speed of 40-70 m/sec., so as to obtain finely dispersed calcium hydroxide, (5) terminating the said high energy high shear agitation on achieving finely dispersed slaked lime, (6) carbonating the finely dispersed slaked lime by passing therethrough sufficient gas comprising carbon dioxide to neutralize the pH of the suspension during said carbonation step, (7) subjecting the suspension to continuous agitation with an impeller speed of 200-700 cm/sec to maintain the suspension, and (8) separating the precipitated calcium carbonate formed in the process. The disadvantage associated with this method is requirement for generating high energy high shear agitation during slaking and carbonation.
The use of additives to control the morphology and particle size is also reported. Bleakley Ian S. et al. (U.S. Pat. No. 5,558,850) disclose a process wherein 0.1 to 2.0% by weight of a reagent having one or more active hydrogen atoms e.g., polyhydric alcohol or phenol is added to the aqueous medium in which the quick lime is slaked. Chapnerkar Vasant D. et al. (U.S. Pat. No. 5,332,564) disclose a process wherein quicklime is slaked in an aqueous solution containing about 0.1 to 2.0%, by weight of a sugar for the production of rhombic shaped precipitated calcium carbonate. Bleakley Ian S. et al. (U.S. Pat. No. 5,232,678), disclose a process wherein 0.01 to 1.5% by weight of triethanolamine, mannitol, morpholine and solid boroheptonate are employed in the preparation of clusters of calcium carbonate which give good light scattering properties when used as a paper filler or paper coating pigment. Vanderheiden, Denis B. (U.S. Pat. No. 4,714,603) discloses the use of polyphosphates in an amount of 0.1 to 1.0% by weight for generating precipitated calcite of substantially spherical morphology suitable for use in dull finish coated paper. The disadvantage associated with all these processes is the requirement of special reagents which add to the production cost.
Bleakley Ian S. et al. (U.S. Pat. No. 5,833,747), disclose a method for preparing precipitated calcium carbonate for use as a pigment in paper coating compositions. The method comprises the steps of (1) carbonating an aqueous medium containing lime, (2) at least partially dewatering the precipitated calcium carbonate-containing suspension using a pressure filter device operating at a pressure of 5 to 10 MPa and (3) subjecting the precipitated calcium carbonate-containing suspension to comminution by high shear attrition grinding with an attrition grinding medium such as silica sand having a median particle diameter in the range 0.1 to 4.0 mm. The product predominantly comprises aragonitic or scalenohedral crystals. The disadvantages of this method include the requirement of a device for high shear attrition grinding with a special grinding medium and a pressure filter device for dewatering the precipitated calcium carbonate containing suspension. Also, the grinding medium is not separated during the process.
Kroc Vicki J. et al., disclose a process (U.S. Pat. No. 5,695,733) that comprises the steps of (1) forming a reaction mixture containing seed material of a scalenohedral particles of aragonite type calcium carbonate and (2) adding lime slurry into the reaction mixture while simultaneously introducing carbon dioxide. The flow rates of the lime slurry and carbon dioxide are adjusted to control the solution conductivity of the reaction mixture to from 2 to 4 milli Siemens to form the clusters of calcite particles. The drawback of this process is that it requires simultaneous addition of lime slurry and carbon dioxide to maintain the solution conductivity. Moreover, simultaneous control of flow rates of both liquid phase and gaseous phase reactants is difficult.
You Kyu Jae discloses a process for producing calcium carbonate particles having an average size of 0.1 to 1.0 micron (U.S. Pat. No. 5,811,070). The process comprises the following steps. (1) Carbon dioxide is introduced into a milk of lime containing a first reagent, consisting of sodium glutamate, sugar, or a mixture thereof, to prepare an aqueous suspension containing calcium carbonate particles of 0.4 micron in average size. The concentration of the first reagent is from 0.1 to 2.0 parts per 100 parts of calcium hydroxide initially present in the milk of lime. (2) A milk of lime is added into the above aqueous suspension. (3) A carbonated solution is added to the aqueous suspension, which contains a second reagent comprising at least one of sodium polyacrylate and a bicarbonate in the range of 0.1 to 5.0 parts per 100 parts of calcium hydroxide present initially, Calcium carbonate particles produced by the process are suitable as a filler for adhesives, paints, inks, papers and plastics, especially transparent polyethylene films. The drawback associated with this process is the addition of two different types of reagents in two stages, which makes the process complicated and unattractive.
Vanderheiden discloses a process (U.S. Pat. No. 4,367,207). for preparing finely divided precipitated calcite. In the Vanderheiden process, carbon dioxide is introduced into an aqueous calcium hydroxide slurry containing anionic organo-polyphosphate polyelectrolyte at a temperature from about 7xc2x0 C. to about 18xc2x0 C. One disadvantage of this process is the requirement of an anionic polyelectrolyte which adds to the production cost. Another disadvantage is the required maintenance of a reaction temperature below ambient temperature. This requirement necessitates a chilling plant which is energy consuming.
The present invention provides a process for the production of precipitated calcium carbonate from calcium carbonate-rich by-product generated in a chemical processing industry which process obviates the drawbacks detailed above. Precipitated calcium carbonate is produced by calcination of calcium carbonate-rich by-product into quick lime in a kiln. This is followed by slaking or hydration to obtain hydrated lime. The hydrated lime is subsequently subjected to carbonation.
The present invention also develops an alternative to limestone as a source for producing precipitated calcium carbonate useful for commercial applications.
The present invention also develops a process for purification, calcination, slaking and carbonation of a calcium carbonate-rich by-product having particle size from 20 to 150 microns to produce precipitated calcium carbonate of particle size less than 20 microns.
The present invention also provides a continuous process for the purification of a calcium carbonate rich by-product of a nitrophosphate fertilizer plant thereby minimizing solid waste produced by such a plant.
The present invention also provides pollution abatement measure for a nitrophosphate fertilizer plant by utilizing the calcium carbonate-rich by-product generated in such a plant.
The present invention also produces from calcium carbonate rich by-product generated in a nitrophosphate fertilizer plant, a high-value finely divided precipitated calcium carbonate which is useful as a filler in paints, and in plastics, rubber, poly vinyl chloride (PVC) and paper.
The present invention relates to a process for the production of precipitated calcium carbonate from calcium carbonate-rich by-product generated in industrial processes, specifically from a nitrophosphate plant in the fertilizer industry. In one embodiment, the steps of the process comprise:
(1) calcinating a calcium carbonate rich by-product generated in a nitrophosphate fertilizer plant, the by-product having a moisture content up to 25% and a particle size from 20 to 150 microns, in a rotary calciner at a continuous feed rate from 5 to 20 kg/h at a calcination temperature of above 850xc2x0 C. and below about 950xc2x0 C. with a residence time from 60 to 90 minutes, to obtain a calcined material having from 75 to 88% available calcium oxide;
(2) removing water vapor, volatile matter, ammonia, NOx and carbon dioxide during the calcination using a blower and a scrubber;
(3) slaking the calcined material (calcium oxide) in a slaker provided with an agitator rotating at 120 RPM to produce a hydrated lime slurry having a solids concentration in the range from 15 to 23% by weight;
(4) removing heavier and coarse particles from the hydrated lime slurry by wet sieving through a 60 to 100 mesh sieve to form a fine hydrated lime slurry;
(5) diluting the fine hydrated lime slurry to a solids concentration from 10 to 20% by weight;
(6) transferring the diluted lime slurry to a carbonation tower and passing a carbon dioxide-air mixture containing 25% by volume carbon dioxide at a superficial gas velocity of from 10 to 15 cm/sec at a maintained temperature from 25 to 45xc2x0 C. until the pH of the diluted lime slurry is lowered to near neutral;
(7) separating the precipitates formed in step (6) by known methods, e.g., filtration or centrifugation;
(8) drying and pulverizing the separated precipitates to produce a precipitated calcium carbonate; and (9) optionally, treating the product slurry before the separation with a fatty acid or a salt of a fatty acid such as, for example, stearic acid or sodium stearate at 95xc2x0 C. in a concentration range from 2 to 3.5% so as to obtain a coated precipitated calcium carbonate which is industrially useful in a number of applications such as rubber, plastics, paints and PVC.
The invention is further directed to a precipitated calcium carbonate product having a mean particle size from about 4 to 6 microns wherein 100% of the particles are less than 20 microns and having a weight percent of calcium carbonate greater than 97.
The process described herein significantly purifies a calcium carbonate-rich by-product generated in a nitrophosphate fertilizer plant to produce precipitated calcium carbonate.